Set metadata using the proper method

Co-authored-by: Frédéric Collonval <fcollonval@users.noreply.github.com>

python/jupyterlab_widgets/src/manager.ts

@@ -455,7 +455,7 @@ export class WidgetManager extends LabWidgetManager {
  */
  private _saveState(): void {
  const state = this.get_state_sync({ drop_defaults: true });
- this._context.contentsModel?.content.metadata.set('widgets', {
+ this._context.model.setMetadata('widgets', {
  'application/vnd.jupyter.widget-state+json': state,
  });
  }